This invention relates to proximity detectors and switches, and in one embodiment it is concerned with a touch detector for mounting inside a store window, enabling a user outside the window to control appliances and/or displays inside the store.
A number of different types of proximity detectors and switches have been in use or disclosed. Switch panels that use the reflection of infrared radiation to detect finger touch are disclosed in Sauer's U.S. Pat. No. 4,340,813. Short range reflective controls such as Micro Switch (a division of Honeywell) Model FE7B use the reflection of modulated infrared radiation to detect objects. Fukuyama et al. U.S. Pat. No. 4,306,147 discloses a reflection-type photoelectric switching apparatus which detects the presence of an object in a mechanically adjustable sensing area. Aromat (a member of Matsushita Group) Model MQ-W3A-DC12-24V photoelectric sensor detects the presence of an object in an electrically adjustable sensing area using optical triangulation.
Philipp U.S. Pat. No. 4,879,461 discloses a reflective-type detection system which senses the presence of an object against a background using a general technique of nulling to negate the effects of stray (e.g. ambient) light by adding the complement of the detected signal to a summing point in the circuit to cancel the signal generated by stray light. In a system using a modulated emitter, this requires the generation of an amplitude and phase regulated modulated signal as a complementing signal. As explained below in the present system a filtered and rectified version of a detected signal is nulled with a constant (DC) level signal.
Reflecting-type infrared switches use the reflection of infrared light off an object to detect the presence of the object. Such sensors are in common use in industry to detect the presence of an object without having mechanical contact with the object, leaving the object undisturbed. These sensors lack mechanical contacts which wear and soil, giving them long lifetimes.
Reflective-type infrared switches have the potential to detect finger proximity through glass. A preferred application of a reflecting-type infrared switch is a touch switch that allows the control of electric devices located inside a store window from outside. Locating all system components inside the store window affords them safety from the elements of weather and vandals and eliminates the need to drill through building walls and glass. The operation of electric devices could occur during or after store hours.
Detecting finger proximity through one or more glass panes that might include air gaps in an environment that can include a multitude of radiation sources and reflections increases the difficulty of the task. Reflective-type infrared switches that use modulated emitters and infrared and electronic filters minimize the effect of extraneous radiation outside the bandwidth of the infrared and modulation frequencies. Extraneous radiation sources would include sunlight, artificial window and street lighting, and automobile headlights.
A common detection practice used to detect the presence of an object with a reflecting-type infrared switch using a modulated emitter is to optically and electronically filter the reflected signal and compare it to a threshold which is applied against a composite signal which may include reflections from other objects in the sensing region.
This technique does not take into account the presence and effect of large extraneous reflections which can occur in the preferred window application described above. The present invention addresses this situation by including a means of applying a threshold to an amplified version of the detection signal after the extraneous steady-state signal has been cancelled.
Another detection practice used by Fukuyama and Aromat adjusts the sensing area to exclude extraneous objects from a detection range. This is not adequate in the preferred application since extraneous objects (glass panes) are in intimate proximity to the object to be detected (finger).
Proximity detectors are used in industry to measure, for example, the web tension and accumulating roll diameter of winding fabric, proximity of microprobes from the surface of electronic parts, product width or position determination, and distance measuring on automatic vehicles. Typically ultrasonics are used in applications that resolve within a centimeter. A reflective-type proximity detector offers the possibility of measuring displacement to a much finer resolution, albeit over a much shorter working distance, typically under 30 centimeters.
Reflecting-type infrared proximity detectors operate on principles similar to reflective-type proximity switches, having instead an analog signal output indicating the strength of the reflected signal, which increases as the object gets closer to the detector. One difficulty encountered by reflective-type proximity detectors is the tradeoff of sensitivity to incremental movements and saturation from total reflection. This situation is similar to the case of the reflecting-type infrared switch where there is a small change in reflection (due to incremental movement) in the presence of a large background reflection (due to total reflection).
It is therefore a purpose of the invention to create a reflective-type proximity switch that can detect the touch of a human finger, discriminated from stray radiation in a multitude of environments including the preferred application mentioned above. It is a further purpose of the invention to produce an output which is relative and which increases as an object comes closer to the detector, removing the effect of total reflection to allow greater amplification of incremental movement.